In the art of rotary, positive displacement gas compressors, including sliding vane and helical screw types, it is conventional practice to unload the compressor while running at constant speed by throttling the compressor inlet to reduce or shut off inlet gas flow. Such a method of compressor unloading has proved to be simple and reliable but has the disadvantage that power consumption of the compressor while running unloaded or at idle is quite high often on the order of 60-80 percent of full load power consumption. This high power input required during unloaded running is due to the compressor continuing to compress or work against high pressure in the compressor discharge conduit and in the working chambers of the compressor. Internal leakage into the compression chambers of fluid flowing back from the discharge port causes continual recompression thereby requiring substantial power input to the compressor. Moreover, it is usually necessary in compressors which are liquid injected to continue to inject liquid at a substantial rate during unloaded operation to prevent heat buildup from the constant recompression of the working fluid in the compressor. This high liquid injection rate increases the pumping work done by the compressor as well as contributes to the cooling load on the compressor unit at unloaded conditions.
Previous attempts to provide improved control systems for unloading rotary compressors include systems which reduce back pressure in the discharge line by venting the line to atmospheric pressure either by blowing down the reservoir tank downstream of the compressor or by venting the compressor discharge line into an auxiliary receiver at atmospheric pressure. Such systems usually include shutting off of inlet gas flow while the discharge side of the compressor is vented to atmosphere. Such systems have the disadvantage that considerable power is required to compress the gas that backflows into the compressor from the discharge line even though the gas pressure in the discharge port and passages is reduced to atmospheric pressure. Power requirements of such systems during unloaded operation are often on the order of 25 percent of full load power assuming an air compressor working to compress from atmospheric pressure to a discharge pressure of 100 p.s.i. (7.03 Kg/cm.sup.2). U.S. Pat. No. 2,977,039 to W. E. Green et al. and U.S. Pat. No. 3,186,631 to R. E. Lamberton et al. disclose systems generally of the above mentioned type.
U.S. Pat. No. 3,260,444 to R. F. Williams et al. discloses an unloading control system for a liquid injected helical screw compressor in which a pump is connected to the compressor discharge conduit during unloaded operation for evacuating the gas and liquid in the discharge line between a downstream check valve and the compressor proper. In this type of system it is possible to substantially evacuate the compressor discharge conduit and working chambers. However, the system does require sufficient liquid injection to keep the pump sealed, cooled, and lubricated. The amount of liquid needed to maintain proper operation of the pump has generally been in excess of the amount required to lubricate the compressor bearings and rotors and has been found to cause undesirable noise and vibration when injected into the compressor in the manner and quantities required for the Williams et al system. Moreover, the pump itself is not always required for furnishing injection liquid in some compressor systems and therefore in such systems the pump becomes an extra cost item as part of the unloading system.
Accordingly, it has been deemed desirable to provide a compressor control system for unloading gas compressors including liquid injected positive displacement compressors wherein the back pressure or working pressure in the discharge port and the compressor working chambers may be reduced as much as possible without continued injection of copious amounts of liquid and without requiring auxiliary pumping devices.